Prop balancing article
I read the prop balancing article in the October issue of Sport Aviation with interest. I have several fixed pitch props, but all have a 2.25" hole, not .75" as described in the article. I have not found a company that makes a fixture for static balancing props with the larger hole? Is this something people are having custom made by a machine shop? ... as in, several bushings with a rod through the middle? Any advice would be appreciated.
Bainbridge Island, WA
Why do you need a rod the same size as the hole? I used a 3/4" piece of drill rod.
BTW, for the fixture, I just made up a wooden "u" cradle and cut a pair of slots in the top which just fit the knives from my jointer. I (carefully) tapped the knives in the slots, mounted them level and had an almost frictionless surface on which to set the drill rod. The prop was sitting on the drill rod and was very responsive to a heavy blade. Even a little piece of masking tape stuck to one blade would rotate that blade downward.
It would be best to have a bushing for the prop center hole, but just sitting on the drill rod seems to work fine.
What you guys are doing is static balancing, which will often take care of most of inbalance. But not always, especially with longer props and more powerful engines turning higher rpm. Two big examples are Reno unlimited racers like Rare Bear or Strega, and perhaps even more so helicopter blades. These need dynamic balancing, and that is good for any prop. It can often be done for about $150 for many planes and has no downside, all the benefit is to the good. It can really make a differnce, I have tried it on a number of planes.
Can't say for sure, but think static balancing would take care of 90% of any vibration, possibly higher. The rest can be done with dynamic systems, but sometimes thats a real pain to deal with. Army helicopters have dynamic monitoring systems installed, and sometimes you will run into one that has all the dynamic systems balanced, flight controls properly rigged, P/C rods set to within 1/32" of specified limits, etc etc. In other words, as close to theoretically perfect as is possible. Spent a better part of a month trying to figure out why this "perfect" system wouldn't meet track and balance limits.
It was the landing gear.
Static propeller balancing without finding the center of rotation seems like a waste of time to me. Just "hanging" the prop on a piece of drill rod presupposes that the center bore of the prop is smooth, round, and centered on the center of rotation - probably not! The "center" bore might better be considered as a "suggestion" of where the "center" might be. Having experience with wheel balancing can actually be detrimental because the center of your basic bass-boat trailer wheel is far more reliably centered than the center bore of a wooden prop, especially if that prop is not from a certified prop manufacturer. If you want to do the static balance "right" you have to find the center of rotation as it will be when installed, then balance about that point.
But suppose the static balance is "perfect". Then you still have tracking issues and blade angle issues. The tracking can be adjusted to some extent by bolt torque adjustments on installation, but what about blade angle issues? Not all two-bladed propellers are symmetrical blade-for-blade, and the rear face of the hub may not be truly perpendicular to the bisector of the angles between the blades even if the blades measure out as identical to one another. The result is that a "perfectly balanced" propeller may pull harder on one blade than the other, resulting in whirl-mode vibrations in flight. A good dynamic balancer can find and fix most of these types of vibration issues without even unbolting the prop from the hub.
Should one do a static balance of the prop? Absolutely, especially after any maintenence. But be sure to do it "on centers" so that the balance point (Center of Gravity) is coincident with the center of rotation. (Anything less is just pot-luck.)
Having done that, is the static balance going to be "good enough"? Maybe. It depends on your tolerance for vibration. Talk to someone who has done all the static balancing and then gotten a dynamic balance (especially the in-flight variety), and you'll be amazed how forcefully they will insist that anything less than a dynamic balance is just not good enough!
I had a metal prop on a 65 Continental that always had a small vibration. Took it to a prop shop and had it checked & static balanced. The owner told me that the prop must have had contact with a solid object at one time, as you could place the prop blades horizonally and it balanced perfectly, however, if you placed it with the blades vertical, it rotated around, indicating that the blades weren't completely in line with each other. The same blade always rotated down to the bottom when vertical, even though it balanced in the horizontal position. Something to think about when balancing a propeller.
I agree that static balancing gets the prop most of the way there, but dynamic is best. I have a very nice used Pacesetter wood prop that I'd like to use on my airplane. The forward side (back) is like new, but the back side (front) has been touched up (badly), and needs to be refinished. I have hesitated starting this project because I wasn't sure how I am going to static balance it. My young adult son suggested 57mm (approx. 2.25") skate board wheels to use for bushings. Skate board wheels are set up for two sealed bearings, one on each side, and a 5/16" hole in the middle. Yesterday I went to the local skateboard shop with prop in hand, and the two young lads took great interest in my project. We used two 57mm skate board wheels, four bearings, and a 5/16"drill rod (had to run over to Home Depot to purchase). We bench tested the set-up, and there was about .005" run-out, which I attribute to the drill bit being slightly smaller than the hole in the bearings. They gave me the four bearings, and I had to purchase a new set of wheels for $18.95. I tipped them $20 for their time because we spent about 30 minutes on it in their shop. I think a numbered drill bit that fits the inner hole of the bearing would take out most of the run-out. The wheels are made very precisely.
This weekend Iíll get going on the cradle. Thanks very much for all the responses. I really appreciate it.
Sounds like a well thought out idea!
Thanks very much for responding. Yes, I have seen that presented in Contact. If I had a lathe, I wouldn't be playing around with skateboard wheels.